GaMnAsTEM

Présentation

Since ferromagnetism in GaMnAs is mediated by holes released by the Mn
substitutional atoms, the introduction of donors in the matrix is
deleterious. Unfortunately, GaMnAs grown by low-temperature MBE may
contain substantial concentrations of point defects acting as donors,
which it is essential to measure. In addition to substitutional
defects, we considered specifically AsGa antisites and Mn interstitial
atoms, of which there may be two my types, having respectively Ga and
As neighbors.

Fig 1: GaMnAs Unit Cell. In addition to Mn substitutional, two kinds of defects are present: Arsenic antisites on Ga sites, and Mn interstitials. We considered
two types of intersticials, 1 and 2, with either As or Ga neighbors. Our method allows us to identify the first species as the dominant one.

TEM contrast analysis method

We devised an original method for the determination of
the local concentrations of these four species (which vary between a
fraction of % and several %). The experiments are carried out in a TEM.
The method relies on the sensitivity of the structure factors of weak
reflections (for fast electron scattering) to the concentrations and
crystal-lattice locations of these minority constituents. High spatial
resolution is obtained by combining one structure factor measurement
with two X-ray analyzes at the same location in the specimen. These
three measurements are not enough to determine fully the four
concentrations, but assign them to narrow ranges.

Moreover, simply
examining the image contrast with respect to the GaAs substrate is
sufficient to determine which type of Mn interstitial dominates. We
found that the Mn interstitials having As nearest neighbors prevail in
all the as-grown layers examined. It is highly unusual for a TEM method
to be capable of determining the concentrations and the lattice
locations of point defects.